Cleaning medium for magnetic recording devices

ABSTRACT

A cleaning medium for magnetic recording devices comprises a non-magnetic substrate, a lower coating layer, which is overlaid upon the non-magnetic substrate and primarily contains a binder and non-magnetic inorganic particles dispersed in the binder, and a cleaning layer, which is overlaid upon the lower coating layer and contains a binder and inorganic particles dispersed in the binder and at least containing ferromagnetic particles. An at least 50% by weight portion of the non-magnetic inorganic particles, which are contained in the lower coating layer, is constituted of granular inorganic particles, which have a mean particle diameter of at most 0.08 μm, or acicular inorganic particles, which have a mean longer axis length falling within the range of 0.05 μm to 0.3 μm and an acicular ratio falling within the range of 3 to 20. The cleaning medium removes dirt from magnetic heads with high cleaning power, such that the wear of the magnetic heads may be small and no scratch may occur on the cleaned magnetic heads.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a cleaning medium for magnetic recordingdevices, such as a cleaning tape, for use in cleaning of magnetic headsor movement systems of magnetic recording and reproducing devices, suchas those of audio, video, or computer equipments.

2. Description of the Prior Art

In general, with magnetic recording devices utilized in video, audio, orcomputer equipments, the recording or reproduction of magneticinformation is carried out by moving a magnetic medium, such as amagnetic tape, while it is being in sliding contact with a magnetichead. At this time, if chips having been scraped out from the magnetictape, dust contained in the ambient air around the device, or the like,clings to the surface of the magnetic head, the reproduction output willbecome low and, in the worst case, no output can be obtained. In suchcase, a cleaning medium, such as a cleaning tape, is utilized in orderto remove dirt from the surface of the magnetic head and to therebyrestore the desired level of the reproduction output.

Nowadays, there is a strong demand for the recording of magneticinformation at high densities. In the cases of the recording of magneticinformation at high densities, the problems often occur in that, if thestate of contact between the end of the magnetic head and the magnetictape is not kept good, the recording and the reproduction of magneticinformation cannot be carried out satisfactory. Therefore, besides theremoval of dirt from the magnetic head, it is necessary for the cleaningtape to have the functions for trueing up the shape of the magnetic headto a certain extent. In particular, recently, magnetic heads areutilized in magnetic recording and reproducing devices for carrying outthe recording of magnetic information at high densities such that theshortest recording wavelength may be at most 1 μm. As for such magneticheads, the gap length is at most 0.4 μm, and therefore even slight dirton the magnetic heads will adversely affect the recording andreproducing performance.

Also, as the material of magnetic heads, ferrite having a high surfacehardness was popular in the past. However, recently, in order to achievethe recording of magnetic information at high densities, metal heads,such as Sendust heads, are used widely. The metal heads are softer andsuffer from more wear than the ferrite heads. Therefore, it is importantthat the amount of wear of the magnetic heads due to the cleaning tapebe kept small.

As for the abrasive tape for use in the polishing of magnetic heads, anabrasive tape, which comprises a non-magnetic substrate, an intermediatelayer containing non-magnetic particles, and an abrasive layercontaining non-magnetic abrasive particles, the intermediate layer andthe abrasive layer being overlaid in this order on the non-magneticsubstrate, has been disclosed in, for example, Japanese UnexaminedPatent Publication No. 62(1987)-92205. The disclosed abrasive tapeemploys the two-layer constitution, in which the surface of theintermediate layer is rendered rough, and in which the diameter of theabrasive particles contained in the abrasive layer overlaid upon theintermediate layer is set to be small, such that a desired level ofpolishing power may be obtained and the magnetic head may be preventedfrom being scratched. However, the disclosed abrasive tape is the onedesigned for the finishing polishing, and its surface roughness fallswithin the range of 0.03 μm to 0.3 μm. Therefore, the disclosed abrasivetape yields much wear of magnetic heads, is liable to cause scratches tooccur on magnetic heads, and is therefore not suitable for use as acleaning tape. Abrasive tapes for magnetic heads, which are similar tothe abrasive tape disclosed in the aforesaid publication, are proposedin, for example, Japanese Unexamined Patent Publication Nos.62(1987)-94270 and 62(1987)-92205. Each of the proposed abrasive tapesis provided with an abrasive layer constituted of two layers. However,the proposed abrasive tapes are not suitable for carrying out thecleaning with strong cleaning power such that the magnetic heads may notbe scratched.

As for the cleaning tape for magnetic heads, a cleaning tape aiming atsatisfying the two requirements with respect to good cleaningcharacteristics and little wear of magnetic heads has been disclosed in,for example, Japanese Unexamined Patent Publication No. 6(1994)-139531.The disclosed cleaning tape comprises a flexible substrate and twocleaning layers overlaid upon the flexible substrate, wherein theYoung's modulus of the upper cleaning layer is set to be larger than theYoung's modulus of the lower cleaning layer. However, the two cleaninglayers of the disclosed cleaning tape contain ferromagnetic particles,and the disclosed cleaning tape cannot sufficiently satisfy the tworequirements in that the cleaning tape should have strong cleaningcharacteristics as a whole and in that the cleaning tape should notcause scratches to occur on magnetic heads.

SUMMARY OF THE INVENTION

The primary object of the present invention is to provide a cleaningmedium for magnetic recording devices for use in cleaning of magneticheads or movement systems of high-density magnetic recording andreproducing devices, or the like, which cleaning medium yields littlewear of magnetic heads, is capable of being used in cleaning metalheads, has good cleaning characteristics for removing dirt from magneticheads with high cleaning power and restoring a desired level ofreproduction output, and causes no scratches to occur on the cleanedmagnetic heads.

The present invention provides a cleaning medium for magnetic recordingdevices, comprising:

i) a non-magnetic substrate,

ii) a lower coating layer, which is overlaid upon the non-magneticsubstrate and primarily contains a binder and non-magnetic inorganicparticles dispersed in the binder, and

iii) a cleaning layer, which is overlaid upon the lower coating layerand contains a binder and inorganic particles dispersed in the binder,the inorganic particles at least containing ferromagnetic particles,

wherein an at least 50% by weight portion of the non-magnetic inorganicparticles, which are contained in the lower coating layer, isconstituted of granular inorganic particles (having granular shapes,that include a spherical shape and polyhedral shapes ranging from anapproximately spherical shape to a cubic shape), which have a meanparticle diameter of at most 0.08 μm, or acicular inorganic particles,which have a mean longer axis length falling within the range of 0.05 μmto 0.3 μm and an acicular ratio falling within the range of 3 to 20.

In the cleaning medium in accordance with the present invention, thenon-magnetic inorganic particles, which are contained in the lowercoating layer, should preferably be constituted of at least a singlekind of particles selected from the group consisting of titanium oxideparticles, α-iron oxide particles, barium sulfate particles, zinc oxideparticles, and alumina particles.

Also, the lower coating layer and the cleaning layer should preferablybe formed with a wet-on-wet coating technique, in which, after the lowercoating layer has been coated on the substrate, the cleaning layer iscoated on the lower coating layer while the lower coating layer is beingin the wet state.

With the cleaning medium in accordance with the present invention, thelower coating layer, which primarily contains the binder and thenon-magnetic inorganic particles dispersed in the binder, is overlaidupon the non-magnetic substrate. Also, the cleaning layer, whichcontains the binder and the inorganic particles dispersed in the binder,is overlaid upon the lower coating layer. The inorganic particles in thecleaning layer at least contain the ferromagnetic particles. An at least50% by weight portion of the non-magnetic inorganic particles, which arecontained in the lower coating layer, is constituted of the granularinorganic particles, which have a mean particle diameter of at most 0.08μm, or the acicular inorganic particles, which have a mean longer axislength falling within the range of 0.05 μm to 0.3 μm and an acicularratio falling within the range of 3 to 20. Therefore, in cases where thecleaning medium in accordance with the present invention is moved alonga magnetic head in order to clean it while the cleaning layer of thecleaning medium is being in sliding contact with the surface of themagnetic head, the magnetic head can be cleaned strongly by the smoothsurface of the cleaning layer. The cleaning medium in accordance withthe present invention has a strong cleaning power and does not scratchthe magnetic head.

In cases where the non-magnetic inorganic particles, which are containedin the lower coating layer, are constituted of the fine granularinorganic particles, which have a mean particle diameter of at most 0.08μm, a smooth lower coating layer, which have good dispersion quality andan isotropic dynamic strength, can be obtained. In cases where thecleaning layer is coated onto the lower coating layer while the lowercoating layer is being in the wet state, a smooth cleaning layer can beformed. As a result, the layer having a high strength in the verticaldirection can be obtained. Accordingly, the smooth cleaning layer canquickly and appropriately clean the magnetic head such that the cleanedmagnetic head may not be scratched.

In cases where the non-magnetic inorganic particles, which are containedin the lower coating layer, are constituted of the fine acicularinorganic particles, which have a small mean longer axis length fallingwithin the range of 0.05 μm to 0.3 μm and a large acicular ratio fallingwithin the range of 3 to 20, a lower coating layer, in which theacicular inorganic particles are well-ordered, can be obtained.Therefore, for example, in cases where the cleaning layer is coated ontothe lower coating layer while the lower coating layer is being in thewet state, a smooth cleaning layer can be formed. As a result, the layerhaving a high strength in the longitudinal direction can be obtained.Accordingly, the smooth cleaning layer can quickly and appropriatelyclean the magnetic head such that the cleaned magnetic head may not bescratched.

DETAILED DESCRIPTION OF THE INVENTION

Basically, the cleaning medium in accordance with the present inventioncomprises a non-magnetic flexible substrate, a lower coating layeroverlaid upon the non-magnetic flexible substrate, and a cleaning layeroverlaid upon the lower coating layer. The lower coating layer primarilycontains a binder and non-magnetic inorganic particles dispersed in thebinder. The cleaning layer contains a binder and inorganic particlesdispersed in the binder, the inorganic particles at least containingferromagnetic particles.

An at least 50% by weight portion of the non-magnetic inorganicparticles, which are contained in the lower coating layer, isconstituted of the granular inorganic particles (having granular shapes,that include a spherical shape and polyhedral shapes ranging from anapproximately spherical shape to a cubic shape), which have a meanparticle diameter of at most 0.08 μm, or the acicular inorganicparticles, which have a mean longer axis length falling within the rangeof 0.05 μm to 0.3 μm and an acicular ratio falling within the range of 3to 20. The non-magnetic inorganic particles, which are contained in thelower coating layer, should preferably be constituted of at least asingle kind of particles selected from the group consisting of titaniumoxide particles, α-iron oxide particles, barium sulfate particles, zincoxide particles, and alumina particles.

The thickness of the cleaning layer should preferably fall within therange of 0.05 μm to 1.0 μm. The surface of the cleaning layer is formedto be smooth, and its surface roughness Ra, expressed in terms ofarithmetic mean deviation, should preferably fall within the range of1.0 nm to 7.0 nm.

The cleaning layer is formed with a wet-on-wet coating technique (asimultaneous or sequential wet coating technique), in which, after thelower coating layer has been coated on the substrate, the cleaning layeris coated on the lower coating layer while the lower coating layer isbeing in the wet state. The thickness of the entire cleaning medium(cleaning tape) should preferably fall within the range of 4 μm to 15μm. The thickness of the substrate should preferably fall within therange of 2 μm to 10 μm, and the thickness of the lower coating layershould preferably fall within the range of 0.2 μm to 5.0 μm.

As for the magnetic characteristics of the cleaning layer, thesquareness ratio (remanent magnetic flux density Br/maximum magneticflux density Bm) in the magnetization curve, as measured under anexternal magnetic field of 10 kOe, falls within the range of 0.6 to0.98, and the coercive force Hc falls within the range of 500 Oe to3,000 Oe. The surface roughness Ra, expressed in terms of arithmeticmean deviation, of the surface of the substrate should preferably fallwithin the range of 0.5 nm to 7.0 nm.

The lower coating layer will hereinbelow be described in detail.

The inorganic particles employed in the lower coating layer are thenon-magnetic particles and may be constituted of inorganic compoundsselected from the group consisting of metal oxides, metal carbonates,metal sulfates, metal nitrides, metal carbides, and metal sulfides. Anat least 50% by weight portion of the non-magnetic inorganic particles,which are contained in the lower coating layer, is constituted of theaforesaid granular inorganic particles or the aforesaid acicularinorganic particles.

Examples of the inorganic compounds include α-alumina having a degree ofalphatization of at least 90%, β-alumina, γ-alumina, θ-alumina, siliconcarbide, chromium oxide, cerium oxide, α-iron oxide, goethite, corundum,silicon nitride, titanium carbide, titanium oxide, silicon dioxide, tinoxide, magnesium oxide, tungsten oxide, zirconium oxide, boron nitride,zinc oxide, calcium carbonate, calcium sulfate, barium sulfate, andmolybdenum disulfide. The above-enumerated inorganic compounds may beused alone, or two or more of them may be used in combination. From thepoint of view of the commercial availability, the cost, the narrowdistribution of the particle size, and the availability of variousfunction imparting means, titanium dioxide, zinc oxide, iron oxide, andbarium sulfate are preferable. Titanium dioxide and α-iron oxide aremore preferable.

The non-magnetic inorganic particles may have an acicular shape orgranular shapes (that include a spherical shape and polyhedral shapesranging from an approximately spherical shape to a cubic shape). Incases where the granular non-magnetic inorganic particles are employed,the particles having a mean particle diameter of at most 0.08 μm shouldpreferably be contained in proportions of at least 50% by weight withrespect to the total amount of the non-magnetic inorganic particles. Incases where the acicular non-magnetic inorganic particles are employed,the particles having a mean longer axis length falling within the rangeof 0.05 μm to 0.3 μm and an acicular ratio falling within the range of 3to 20 should preferably be contained in proportions of at least 50% byweight with respect to the total amount of the non-magnetic inorganicparticles.

The tap density of the non-magnetic inorganic particles may fall withinthe range of 0.05 g/ml to 2 g/ml, and should preferably fall within therange of 0.2 g/ml to 1.5 g/ml. The water content of the non-magneticinorganic particles may fall within the range of 0.1% by weight to 5% byweight, should preferably fall within the range of 0.2% by weight to 3%by weight, and should more preferably fall within the range of 0.3% byweight to 1.5% by weight. The pH value of the non-magnetic inorganicparticles may fall within the range of 2 to 11, and should preferablyfall within the range of 5 to 10. The specific surface area of thenon-magnetic inorganic particles may fall within the range of 1 m² /g to100 m² /g, should preferably fall within the range of 5 m² /g to 70 m²/g, and should more preferably fall within the range of 10 m² /g to 65m² /g. The crystallite size of the non-magnetic inorganic particlesshould preferably fall within the range of 0.004 μm to 0.3 μm, andshould more preferably fall within the range of 0.04 μm to 0.08 μm. Theoil absorption amount, as measured with dibutyl phthalate (DBP), of thenon-magnetic inorganic particles may fall within the range of 5 ml/100 gto 100 ml/100 g, should preferably fall within the range of 10 ml/100 gto 80 ml/100 g, and should more preferably fall within the range of 20ml/100 g to 60 ml/100 g. The specific gravity of the non-magneticinorganic particles may fall within the range of 1 to 12, and shouldpreferably fall within the range of 3 to 6.

The ignition loss of the non-magnetic inorganic particles shouldpreferably be at most 20% by weight, and should more preferably be zero.The Mohs hardness of the non-magnetic inorganic particles shouldpreferably fall within the range of 4 to 10. The roughness factor of thesurface of the non-magnetic inorganic particles should preferably fallwithin the range of 0.8 to 1.5, and should more preferably fall withinthe range of 0.9 to 1.2. The stearic acid (SA) adsorption amount of thenon-magnetic inorganicparticles should preferably fall within the rangeof 1 μmol/m² to 20 mol/m², and should more preferably fall within therange of 2 μmol/m² to 15 μmol/m². The heat of wetting of thenon-magnetic inorganic particles with water at 25° C. should preferablyfall within the range of 200 erg/cm² to 600 erg/cm². Solvents yieldingthe heat of wetting falling within this range may be utilized. Thenumber of water molecules on the particle surface at a temperature of100° C. to 400° C. should preferably fall within the range of 1 to 10molecules/100 A. The pH value at the isoelectric point in water shouldpreferably fall within the range of 3 to 6.

The surfaces of the non-magnetic inorganic particles should preferablybe treated with a surface treating agent selected from the groupconsisting of Al₂ O₃, SiO₂, TiO₂, ZrO₂, SnO₂, Sb₂ O₃, ZnO, and Y₂ O₃.From the point of view of the dispersibility, Al₂ O₃, SiO₂, TiO₂, andZrO₂ are preferable, among which Al₂ O₃, SiO₂, and ZrO₂ are morepreferable. The above-enumerated surface treating agents may be usedalone, or two or more of them may be used in combination. Also, aco-precipitated surface treatment layer may be employed in accordancewith the characteristics of the non-magnetic inorganic particles whichare to be obtained. Alternatively, the particle surfaces may firstly betreated with alumina, and thereafter the surface layers may be treatedwith silica. As another alternative, the particle surfaces may firstlybe treated with silica, and thereafter the surface layers may be treatedwith alumina. Further, the surface treatment layer may be set to beporous in accordance with the characteristics of the non-magneticinorganic particles which are to be obtained. However, ordinarily, thesurface treatment layer should preferably be homogeneous and dense.

Examples of the non-magnetic inorganic particles employed in the lowercoating layer of the cleaning medium in accordance with the presentinvention include Nanotite supplied by Showa Denko K.K.; HIT-100 andZA-G1, which are supplied by Sumitomo Chemical Co., Ltd.; α Hematite,DPN-250, DPN-250BX, DPN-245, DPN-270BX, and DPN-550BX, which aresupplied by Toda Kogyo K.K.; Titanium Oxide TTO-51B, TTO-55A, TTO-55B,TTO-55C, TTO-55S, TTO-55D, SN-100, α Hematite E270, E271, and E300,which are supplied by Ishihara Sangyo Kaisha, Ltd.; STT-4D, STT-30D,STT-30, and STT-65C, which are supplied by Titan Kogyo K.K.; MT-100S,MT-100T, MT-150W, MT-500B, MT-600B, MT-100F, and MT-500HD, which aresupplied by Teika Co.; FINEX-25, BF-1, BF-10, BF-20, and ST-M, which aresupplied by Sakai Chemical Industry Corp.; DEFIC-Y and DEFIC-R, whichare supplied by Dowa Mining Co., Ltd.; AS2BM and TiO2P25, which aresupplied by Nippon Aerosil Corp.; 100A and 500A, which are supplied byUbe Industries, Ltd.; Y-LOP supplied by Titan Kogyo K.K.; and productsobtained by firing the above-enumerated non-magnetic inorganicparticles.

Among the above-enumerated non-magnetic inorganic particles, α-ironoxide and titanium dioxide are particularly preferable. As the α-ironoxide (hematite), the particles obtained under the conditions describedbelow may be used. Specifically, acicular goethite particles serving asthe precursor of the α-Fe₂ O₃ particles are obtained with one of themethods described below. In a first method, at least an equivalentamount of an aqueous alkali hydroxide solution is added to an aqueousferrous iron solution, and a suspension containing ferrous hydroxidecolloid is thereby obtained. Thereafter, the pH value of the suspensionis set to be at least 11, and an oxygen-containing gas is introducedinto the suspension at a temperature of at most 80° C. The oxidationreaction is thus carried out, and the acicular goethite particles arethereby obtained. In a second method, an aqueous ferrous salt solutionand an aqueous alkali carbonate solution are reacted with each other,and a suspension containing FeCO₃ is thereby obtained. Thereafter, anoxygen-containing gas is introduced into the suspension. The oxidationreaction is thus carried out, and spindle-shaped goethite particles arethereby obtained. In a third method, an amount of an aqueous alkalihydroxide solution or an aqueous alkali carbonate solution, which amountis less than the equivalent amount, is added to an aqueous ferrous saltsolution, and an aqueous ferrous salt solution containing ferroushydroxide colloid is thereby obtained. An oxygen-containing gas is thenintroduced into the aqueous ferrous salt solution, which contains theferrous hydroxide colloid. The oxidation reaction is thus carried out,and acicular goethite nuclear particles are thereby formed. Thereafter,an amount of an aqueous alkali hydroxide solution, which amount is atleast equivalent with respect to Fe²⁺ contained in the aqueous ferroussalt solution containing the acicular goethite nuclear particles, isadded to the aqueous ferrous salt solution containing the aciculargoethite nuclear particles. An oxygen-containing gas is then introducedinto the resulting mixture, and the acicular goethite nuclear particlesare thereby grown. In a fourth method, an amount of an aqueous alkalihydroxide solution or an aqueous alkali carbonate solution, which amountis less than the equivalent amount, is added to an aqueous ferrous ironsolution, and an aqueous ferrous salt solution containing ferroushydroxide colloid is thereby obtained. An oxygen-containing gas is thenintroduced into the aqueous ferrous salt solution, which contains theferrous hydroxide colloid. The oxidation reaction is thus carried out,and acicular goethite nuclear particles are thereby formed. Thereafter,the acicular goethite nuclear particles are grown under acidic toneutral conditions. During the reaction for forming the goethiteparticles, different elements, such as Ni, Zn, P, and Si, which areordinarily employed in order to improve the characteristics of theparticles, may be added to the reaction mixture.

Thereafter, the acicular goethite particles, which serve as theprecursor particles, are dehydrated at a temperature falling within therange of 200° C. to 500° C. When necessary, the goethite particles maythen be annealed with heat treatment at a temperature falling within therange of 350° C. to 800° C. In this manner, the acicular α-Fe₂ O₃particles are obtained. The anti-sintering agents, such as P, Si, B, Zr,and Sb, may be adhered to the surfaces of the acicular goethiteparticles, which are subjected to the dehydration or the annealing. Asdescribed above, the annealing may be carried out with heat treatment ata temperature falling within the range of 350° C. to 800° C.Specifically, with the annealing, the pole surfaces of the acicularα-Fe₂ O₃ particles having been obtained from the dehydration can befused, and pores having occurred on the particle surfaces can thereby beclosed. As a result, the particles having smooth surfaces can beobtained.

The acicular α-Fe₂ O₃ particles having been obtained from thedehydration or the annealing are then dispersed in an aqueous solution,and a suspension is thereby obtained. An Al compound is added to thesuspension, and the pH value of the suspension is adjusted. The surfacesof the α-Fe₂ O₃ particles are thereby covered with the Al compound.Thereafter, filtration, washing with water, drying, and grinding arecarried out. Also, when necessary, deaeration, consolidation treatment,or the like, may be carried out. In this manner, the α-Fe₂ O₃ particles,which may be employed in the present invention, can be obtained. As theAl compound, it is possible to employ an aluminum salt, such as aluminumacetate, aluminum sulfate, aluminum chloride, or aluminum nitrate; or analkali aluminate, such as sodium aluminate. The proportion of the Alcompound, as calculated in terms of Al, may fall within the range of0.01% by weight to 50% by weight with respect to the α-Fe₂ O₃ particles.If the proportion of the Al compound, as calculated in terms of Al, isless than 0.01% by weight with respect to the α-Fe₂ O₃ particles,dispersion in the binder resin cannot be carried out sufficiently. Ifthe proportion of the Al compound, as calculated in terms of Al, is morethan 50% by weight with respect to the α-Fe₂ O₃ particles, adverseeffects will occur from the interaction between the portions of the Alcompound floating on the particle surfaces.

The non-magnetic inorganic particles, which are contained in the lowercoating layer of the cleaning medium in accordance with the presentinvention, may be covered with the Al compound together with at least asingle kind of compound selected from the group consisting of Sicompounds, P compounds, Ti compounds, Mn compounds, Ni compounds, Zncompounds, Zr compounds, Sn compounds, and Sb compounds. The proportionof each of the compounds, which may be employed together with the Alcompound, may fall within the range of 0.01% by weight 50% by weightwith respect to the α-Fe₂ O₃ particles. If the proportion of each of thecompounds, which may be employed together with the Al compound, is lessthan 0.01% by weight with respect to the α-Fe₂ O₃ particles, the effectsfor improving the dispersibility by the addition of the compound cannotbe obtained sufficiently. If the proportion of each of the compounds,which may be employed together with the Al compound, is more than 50% byweight with respect to the α-Fe₂ O₃ particles, adverse effects willoccur from the interaction between the portions of the compound floatingin the region other than the particle surfaces.

Titanium dioxide, which may be employed as the non-magnetic inorganicparticles in the lower coating layer, may be produced in the mannerdescribed below. Specifically, titanium dioxide may be produced with asulfuric acid process or a chlorine process. With the sulfuric acidprocess, an ilmenite raw ore is digested with sulfuric acid, and Ti, Fe,and the like, are extracted as sulfates. Iron sulfate is then separatedby crystallization and removed. The remaining titanyl sulfate solutionis purified by filtration and subjected to thermo-hydrolysis. Hydroustitanium oxide is thereby precipitated. The hydrous titanium oxidecollected by filtration and washed, and impurities are removed therefromby washing. A particle diameter regulating agent, or the like, is thenadded to the hydrous titanium oxide, and the resulting mixture is firedat a temperature falling within the range of 80° C. to 1,000° C. Crudetitanium oxide is thereby obtained. The rutile-type titanium oxide orthe anatase-type titanium oxide is obtained, depending upon the kind ofthe nucleating agent, which is added at the time of the hydrolysis. Thecrude titanium oxide is subjected to grinding, dressing of particles,surface treatment, and the like, and the desired titanium oxide isthereby obtained. With the chlorine process, natural rutile or syntheticrutile is employed as the raw ore. The ore is chlorinated underhigh-temperature reducing conditions. By the chlorination, Ti isconverted into TiCl₄, and Fe is converted into FeCl₂. Iron oxide havingbeen solidified by cooling is separated from TiCl₄. The thus obtainedcrude TiCl₄ is purified by rectification, and a nucleating agent is thenadded to the purified TiCl₄. Thereafter, TiCl₄ is instantaneouslyreacted with oxygen at a temperature of at least 1,000° C., and crudetitanium oxide is thereby obtained. The finishing method for impartingthe pigment-like properties to the crude titanium oxide having beenformed in the oxidation and decomposition step is the same as thatemployed in the sulfuric acid process.

Surface treatment is carried out in the manner described below.Specifically, after the aforesaid titanium oxide material is groundunder dry conditions, water and dispersing agent are added to thetitanium oxide material, and the resulting mixture is subjected to wetgrinding and centrifugal separation. Coarse particles are therebyclassified. Thereafter, the fine particle slurry is introduced into asurface treatment tank, and the surfaces of the particles are coveredwith a metal hydroxide. Firstly, a predetermined amount of an aqueoussolution of a salt of Al, Si, Ti, Zr, Sb, Sn, Zn, or the like, is addedto the fine particle slurry, and an acid or an alkali for neutralizationis added. The surfaces of the titanium oxide particles are therebycovered with the thus formed hydrous oxide. Water-soluble salts formedas by-products are removed by decantation, filtration, and washing. ThepH value of the slurry is adjusted, the slurry is then filtrated, andwashed with deionized water. Thereafter, the washed cake is dried with aspray dryer or a band dryer. The dried material is then ground with ajet mill, and the final product is thereby obtained. Instead of surfacetreatment being carried out in the water-based system, vapor of AlCl₃ orSiCl₄ may be passed over the titanium oxide particles, steam may then beintroduced, and the gain surfaces may thereby be treated with Al or Si.As for the other pigment producing techniques, reference may be made to"Characterization of Powder Surfaces" by G. D. Parfitt and K. S. W.Sing, Academic Press, 1976.

The non-magnetic inorganic particles described above may also containcarbon black. The binder, in which the non-magnetic inorganic particlesare to be dispersed, will be described later.

In cases where carbon black is contained in the lower coating layer, asis already known, the antistatic effects can be obtained by reducing theelectrical surface resistance Rs, and the light transmittance can bekept small. Also, a desired micro Vickers hardness can be obtained. Themicro Vickers hardness may ordinarily fall within the range of 25 kg/mm²to 60 kg/mm². Such that the state of contact with a magnetic head may beadjusted, the micro Vickers hardness should preferably fall within therange of 30 kg/mm² to 50 kg/mm². The micro Vickers hardness is measuredwith a thin film hardness tester HMA-400 supplied by NEC Co., Ltd. byusing a diamond stylus having a triangular pyramid shape, which has adihedral angle of 80 degrees and an end radius of 0.1 μm, at the end ofthe penetrator.

As the carbon black, furnace black for rubber, thermal black for rubber,coloring black, acetylene black, or the like, may be used. The specificsurface area of the carbon black may fall within the range of 100 m² /gto 500 m² /g, and should preferably fall within the range of 150 m² /gto 400 m² /g. The dibutyl phthalate (DBP) oil absorption of the carbonblack may fall within the range of 20 ml/100 g to 400 ml/100 g, andshould preferably fall within the range of 30 ml/100 g to 200 ml/100 g.The primary particle diameter of the carbon black may fall within therange of 10 nm to 80 nm, should preferably fall within the range of 10nm to 50 nm, and should more preferably fall within the range of 10 nmto 40 nm. The carbon black should preferably have a pH value fallingwithin the range of 2 to 10, a water content falling within the range of0.1% to 10%, and a tap density falling within the range of 0.1 g/ml to 1g/ml.

Examples of the carbon black capable of being used in the lower coatinglayer of the cleaning medium in accordance with the present inventioninclude Blackpearls 2000, 1300, 1000, 900, 800, 880, 700, and VulcanXC-72, which are supplied by Cabot Co.; #3050B, 3150B, 3250B, #3750B,#3950B, #950, #650B, #970B, #850B, MA-600, MA-230, #4000, and #4010,which are supplied by Mitsubishi Chemical Industries Ltd.; Conductex SC,Raven 8800, 8000, 7000, 5750, 5250, 3500, 2100, 2000, 1800, 1500, 1255,and 1250, which are supplied by Columbian Carbon Co.; and Ketjen BlackEC supplied by Ketjen Black International Company. The carbon black maybe subjected to surface treatment with a dispersing agent, or the like,or maybe grafted with a resin. It is also possible to employ a carbonblack having been treated such that a portion of the carbon blacksurface may be graphitized. Further, before being added to a coatingmaterial, the carbon black may be dispersed in a binder.

The proportion of the carbon black may fall within the range of 5% byweight to 49% by weight with respect to the non-magnetic inorganicparticles described above and may be at most 40% with respect to thetotal weight of the non-magnetic layer. The carbon black materialsdescribed above may be used alone, or two or more of them may be used incombination. As for the carbon black which may be employed in thecleaning medium in accordance with the present invention, reference maybe made to, for example, "Carbon Black Handbook," published by CarbonBlack Society.

Also, organic particles may be added to the lower coating layer inaccordance with the characteristics of the lower coating layer which isto be obtained. Examples of the materials of the organic particlesinclude an acryl styrene resin, a benzoguanamine resin, a melamineresin, and a phthalocyanine pigment. It is also possible to employparticles of a polyolefin resin, a polyester resin, a polyamide resin, apolyimide resin, or a polyfluoroethylene resin. The above-enumeratedorganic particles may be produced with one of methods described in, forexample, Japanese Unexamined Patent Publication Nos. 60(1985)-255827 and62(1987)-18564.

As for the binders, lubricating agents, dispersing agents, additives,solvents, dispersing methods, and other techniques, which may beemployed for the lower coating layer, those ordinarily utilized formagnetic layers may be utilized. In particular, as for the proportionsand kinds of the binders, additives, and dispersing agents, thetechniques already known for magnetic layers may be employed.

The cleaning layer, which is overlaid upon the lower coating layer, willhereinbelow be described in detail.

The inorganic particles, which are employed in the cleaning layer,contain the ferromagnetic particles. As the ferromagnetic particles, itis possible to employ any of already known ferromagnetic particles, suchas the particles of γ-FeO_(x) (where x=1.33 to 1.5), Co-modifiedγ-FeO_(x) (where x=1.33 to 1.5), a ferromagnetic alloy containing α-Fe,Ni, or Co as the main constituent (in proportions of at least 75%),barium ferrite, or strontium ferrite. Among the above-enumeratedferromagnetic particles, the particles of the ferromagnetic alloycontaining α-Fe as the main constituent are preferable. Besides thepredetermined atoms, the ferromagnetic particles may also contain Al,Si, S, Sc, Ca, Ti, V, Cr, Cu, Y, Mo, Rh, Pd, Ag, Sn, Sb, Te, Ba, Ta, W,Re, Au, Hg, Pb, Bi, La, Ce, Pr, Nd, P, Co, Mn, Zn, Ni, Sr, B, and Mgatoms. Particularly, in the cases of metal magnetic materials, Al, Si,Ca, Y, Ba, La, Nd, Co, Ni, and B are important as the elements containedbesides α-Fe. Among the above-enumerated elements, Si, Al, and Y areimportant as the surface treatment agents and the anti-sintering agents.The proportion of Co should preferably fall within the range of 2% byweight to 40% by weight with respect to Fe. The proportion of each ofSi, Al, and Y may fall within the range of 0 to 10% by weight. Beforebeing subjected to the dispersing step, the ferromagnetic particlesmaybe treated with dispersing agents, lubricating agents, surface-activeagents, antistatic agents, or the like, which will be described later.Specifically, various ferromagnetic particles are described in, forexample, Japanese Patent Publication Nos. 44(1969)-14090,45(1970)-18372, 47(1972)-22062, 47(1972)-22513, 46(1971)-28466,46(1971)-38755, 47(1972)-4286, 47(1972)-12422, 47(1972)-17284,47(1972)-18509, 47(1972)-18573, 39(1964)-10307, 48(1973)-39639, and U.S.Pat. Nos. 3,026,215, 3,031,341, 3,100,194, 3,242,005, and 3,389,014.

Among the above-enumerated ferromagnetic particles, the ferromagneticalloy particles may contain small amounts of hydroxides or oxides. Theferromagnetic alloy particles produced in accordance with any ofconventional producing techniques may be employed. Examples of theconventional techniques for producing the ferromagnetic alloy particlesinclude the following:

(a) A technique for reducing a composite organic acid salt (mainly,oxalic acid salt) with a reducing gas, such as hydrogen gas.

(b) A technique for reducing iron oxide with a reducing gas, such ashydrogen gas, and thereby obtaining Fe particles or Fe--Co particles.

(c) A technique for thermally decomposing a metal carbonyl compound.

(d) A technique for adding a reducing agent, such as sodium boronhydride, hypophosphite, or hydrazine, to an aqueous solution of a saltof a ferromagnetic metal, and reducing the salt of the ferromagneticmetal.

(e) A technique for evaporating a metal in an inert gas at a lowpressure, and thereby obtaining fine metal particles.

The ferromagnetic alloy particles having been obtained in the mannerdescribed above may be subjected to known gradual oxidation treatment.Specifically, the ferromagnetic alloy particles may be dipped in anorganic solvent and then dried. Alternatively, the ferromagnetic alloyparticles may be dipped in an organic solvent, an oxygen-containing gasmay then be passed over the particles, oxide films may thereby be formedon the particle surfaces, and the particles may then be dried. Asanother alternative, instead of an organic solvent being used, oxidefilms may be formed on the particle surfaces by adjusting the partialpressures of an oxygen gas and an inert gas.

The specific surface area, as measured with the BET method, of theferromagnetic particles, which are contained in the cleaning layer ofthe cleaning medium in accordance with the present invention, may fallwithin the range of 45 m² /g to 80 m² /g, and should preferably fallwithin the range of 50 m² /g to 70 m² /g. If the specific surface areaof the ferromagnetic particles is less than 40 m² /g or is more than 80m² /g, good surface smoothness cannot be obtained. The crystallite sizeof the ferromagnetic particles, which are contained in the cleaninglayer, may fall within the range of 100 A to 300 A, should preferablyfall within the range of 100 A to 250 A, and should more preferably fallwithin the range of 140 A to 200 A.

The value of saturation magnetization σS of the ferromagnetic particlesshould preferably fall within the range of 100 emu/g to 180 emu/g,should more preferably fall within the range of 110 emu/g to 170 emu/g,and should most preferably fall within the range of 125 emu/g to 160emu/g. The coercive force Hc of the ferromagnetic particles shouldpreferably fall within the range of 500 Oe to 3,000 Oe. The squarenessratio should preferably fall within the range of 0.6 to 0.98. The amountof magnetization of the cleaning layer should preferably fall within therange of 0.03 to 0.3 gauss·cm. The acicular ratio of the ferromagneticparticles should preferably fall within the range of 4 to 18, and shouldmore preferably fall within the range of 5 to 12. The water content ofthe ferromagnetic particles should preferably fall within the range of0.01% to 2%. The water content of the ferromagnetic particles shouldpreferably be adjusted appropriately in accordance with the kind of thebinder used.

The pH value of the ferromagnetic particles should preferably adjustedappropriately in accordance with the combination with the binder used.The pH value of the ferromagnetic particles may fall within the range of4 to 12, and should preferably fall within the range of 6 to 10. Whennecessary, the ferromagnetic particles may be subjected to surfacetreatment using a surface treatment agent, such as Al, Si, P, or one ofoxides of them. The proportion of the surface treatment agent may fallwithin the range of 0.1% to 10%. In cases where surface treatment iscarried out, the rate of adsorption of a lubricating agent, such as afatty acid, can be kept to be at most 100 mg/m². It often occurs thatthe ferromagnetic particles contain a soluble inorganic ion, such as Na,Ca, Fe, Ni, or Sr. The content of such a soluble inorganic ion shouldpreferably be substantially zero. However, in cases where the content ofthe soluble inorganic ion is at most 200 ppm, adverse effects will notparticularly occur on the characteristics of the ferromagneticparticles. The amount of pores of the ferromagnetic particles shouldpreferably be as small as possible. Specifically, the amount of pores ofthe ferromagnetic particles should preferably be at most 20% by volume,and should preferably be at most 5% by volume.

As the binders contained in the cleaning layer and the lower coatinglayer of the cleaning medium in accordance with the present invention,thermoplastic resins, thermosetting resins, or reactive resins, whichare already known, or mixtures of two or more of these resins may beused. The thermoplastic resins may have a glass transition temperaturefalling within the range of approximately -100° C. to approximately 150°C., a number-average molecular weight falling within the range ofapproximately 1,000 to approximately 200,000, preferably approximately10,000 to approximately 100,000, and a polymerization degree fallingwithin the range of approximately 50 to approximately 1,000.

Examples of the thermoplastic resins include polymers or copolymerscontaining, as the constituent unit, vinyl chloride, vinyl acetate,vinyl alcohol, maleic acid, acrylic acid, an acrylic ester, vinylidenechloride, acrylonitrile, methacrylic acid, a methacrylic ester, styrene,butadiene, ethylene, vinyl butyral, vinyl acetal, or a vinyl ether.Examples of the thermoplastic resins also include polyurethane resinsand various kinds of rubber-type resins. Examples of the thermosettingresins or the reactive resins include a phenol resin, an epoxy resin, apolyurethane curable resin, a urea resin, a melamine resin, an alkydresin, an acrylic reactive resin, formaldehyde resin, a silicone resin,an epoxy-polyamide resin, a mixture of a polyester resin and anisocyanate prepolymer, a mixture of a polyester polyol and apolyisocyanate, and a mixture of a polyurethane and a polyisocyanate.The above-enumerated resins are described in detail in, for example,"Plastic Handbook" published by Asakura Shoten. It is also possible touse a known electron beam-curing resin in the lower coating layer or thecleaning layer.

Examples of the aforesaid resins and methods for producing them aredescribed in detail in, for example, Japanese Unexamined PatentPublication No. 62(1987)-256219. The above-enumerated resins may be usedalone, or two or more of them may be used in combination. Examples ofpreferable binders include a combination of a polyurethane resin and atleast a single kind of resin selected from the group consisting of avinyl chloride resin, a vinyl chloride-vinyl acetate resin, a vinylchloride-vinyl acetate-vinyl alcohol resin, and a vinyl chloride-vinylacetate-maleic anhydride copolymer; and a mixture of such a combinationand a polyisocyanate. As for the structures of the polyurethane resins,it is possible to employ any of known structures, such as a polyesterpolyurethane, a polyether polyurethane, a polyether polyesterpolyurethane, a polycarbonate polyurethane, a polyester polycarbonatepolyurethane, a polycaprolactone polyurethane, and a polyolefinpolyurethane. When necessary, such that good dispersion properties andgood durability may be obtained, at least a single kind of polar groupmay be introduced into the above-enumerated binders through acopolymerization reaction or an addition reaction. The polar group maybe selected from the group consisting of --COOM, --SO₃ M, --OSO₃ M,--P═O(OM)₂, --O--P═O(OM)₂, wherein M represents a hydrogen atom or analkali metal salt group, --OH, --NR₂, --N⁺ R₃, wherein R represents ahydrocarbon group, an epoxy group, --SH, --CN, sulfobetaine,phosphobetaine, and carboxybetaine. The proportion of the polar groupmay fall within the range of 10⁻¹ to 10⁻⁸ mol/g, and should preferablyfall within the range of 10⁻² to 10⁻⁶ mol/g.

Specifically, examples of the aforesaid binders which may be employed inthe cleaning medium in accordance with the present invention includeVAGH, VYHH, VMCH, VAGF, VAGD, VROH, VYES, VYNC, VMCC, XYHL, XYSG, PKHH,PKHJ, PKHC, and PKFE, which are supplied by Union Carbide Co.; MPR-TA,MPR-TA5, MPR-TAL, MPR-TSN, MPR-TMF, MPR-TS, MPR-TM, and MPR-TAO, whichare supplied by Nisshin Kagaku Kogyo K.K.; 1000W, DX80, DX81, DX82,DX83, and 100FD, which are supplied by Denki Kagaku Kogyo K.K.; MR-104,MR-105, MR110, MR100, and 400X-110A, which are supplied by Nippon ZeonCo., Ltd.; Nipporan N2301, N2302, and N2304, which are supplied byNippon Polyurethane K.K.; Pandex T-5105, T-R3080, T-5201, Burnock D-400,D-210-80, Crisvon 6109, and 7209, which are supplied by Dainippon Inkand Chemicals, Inc.; Vylon UR8200, UR8300, and UR8600, which aresupplied by Toyobo Co., Ltd.; Daiphelamin 402, 5020, 5100, 5300, 9020,9022, and 7020, which are supplied by Dainichi Seika Kogyo K.K.; MX5004supplied by Mitsubishi Kagaku K.K.; Sanprene SP-150, TIM-3003, andTIM-3005, which are supplied by Sanyo Chemical Industries Ltd.; andSaran F310, and F210, which are supplied by Asahi Chemical Industry Co.,Ltd. Among the above-enumerated binders, MR-104, MR110, MPR-TAO, MPR-TA,UR8200, UR8300, and TIM-3005 are preferable.

Proportion of the binder in the cleaning layer may fall within the rangeof 5% by weight to 24% by weight with respect to the inorganic particlescontaining the ferromagnetic particles, and should preferably fallwithin the range of 8% by weight to 22% by weight with respect to theinorganic particles containing the ferromagnetic particles. In caseswhere a vinyl chloride resin is employed, its proportion may fall withinthe range of 5% by weight to 30% by weight. In cases where apolyurethane resin is employed, its proportion may fall within the rangeof 2% by weight to 20% by weight. The proportion of a polyisocyanate mayfall within the range of 2% by weight to 20% by weight. A vinyl chlorideresin, a polyurethane, and a polyisocyanate should preferably be used incombination. In particular, the layer constitution should preferably beset such that the cleaning layer may not contain a polyisocyanate, andthe lower coating layer may contain a polyisocyanate.

In cases where a polyurethane is employed, the polyurethane shouldpreferably have a glass transition temperature falling within the rangeof -50° C. to 100° C., a breaking extension falling within the range of100% to 2,000%, a breaking stress falling within the range of 0.05 kg/cmto 10 kg/cm, and a yield point falling within the range of 0.05 kg/cm²to 10 kg/cm².

The cleaning medium in accordance with the present invention comprisesat least two layers. Therefore, when necessary, the amount of thebinder, the proportion of a vinyl chloride resin, a polyurethane, apolyisocyanate, or the like, in the binder, the molecular weight of eachresin, the proportion of a polar group, physical characteristics of eachresin, or the like, may be varied between the lower coating layer andthe cleaning layer. As for such techniques, any of already knowntechniques may be employed.

Examples of the polyisocyanates include isocyanates, such as tolylenediisocyanate, 4,4'-diphenylmethane diisocyanate, hexamethylenediisocyanate, xylylene diisocyanate, naphthylene-1,5-diisocyanate,o-toluidine diisocyanate, isophorone diisocyanate, and triphenylmethanetriisocyanate. As the polyisocyanates, it is also possible to use theproducts of reactions of the above-enumerated isocyanates andpolyalcohols, and polyisocyanates produced from condensation ofisocyanates. Such polyisocyanates are commercially available under thetrade names of, for example, Coronate L, Coronate HL, Coronate 2030,Coronate 2031, Myrionate MR, and Myrionate MTL, which are supplied byNippon Polyurethane K.K.; Takenate D-102, Takenate D-11ON, TakenateD-200, and Takenate D-202, which are supplied by Takeda ChemicalIndustries, Ltd.; Desmodur L, Desmodur IL, Desmodur N, and Desmodur HL,which are supplied by Sumitomo Bayer K.K.); and Burnock D502, which issupplied by Dainippon Ink and Chemicals, Inc. In each of the lowercoating layer and the cleaning layer, these polyisocyanates may be usedalone, or a mixture of two or more of them may be used by theutilization of differences in curing reaction properties.

As in the lower coating layer, the cleaning layer may contain carbonblack as the inorganic particles. As the carbon black, furnace black forrubber, thermal black for rubber, coloring black, acetylene black, orthe like, may be used. The specific surface area of the carbon black mayfall within the range of 5 m² /g to 500 m² /g, and the dibutyl phthalate(DBP) oil absorption of the carbon black may fall within the range of 10ml/100 g to 400 ml/100 g. The particle diameter of the carbon black mayfall within the range of 5 mμ to 300 mμ, and the pH value of the carbonblack may fall within the range of 2 to 10. The water content of thecarbon black may fall within the range of 0.1% to 10%, and the tapdensity of the carbon black may fall within the range of 0.1 g/cc to 1g/cc. Examples of the carbon black capable of being used in the cleaninglayer of the cleaning medium in accordance with the present inventioninclude Blackpearls 2000, 1300, 1000, 900, 800, 700, and Vulcan XC-72,which are supplied by Cabot Co.; #80, #60, #55, #50, and #35, which aresupplied by Asahi Carbon K.K.; #2400B, #2300, #5, #900, #950, #970,#1000, #30, #40, and #10B, which are supplied by Mitsubishi ChemicalIndustries Ltd.; Conductex SC, Raven 150, 50, 40, and 15, which aresupplied by Columbian Carbon Co. The carbon black maybe subjected tosurface treatment with a dispersing agent, or the like, or may begrafted with a resin. It is also possible to employ a carbon blackhaving been treated such that a portion of the carbon black surface maybe graphitized. Further, before being added to a coating material forthe formation of the cleaning layer, the carbon black may be dispersedin a binder. The carbon black materials described above may be usedalone, or two or more of them may be used in combination.

In cases where the carbon black is contained in the cleaning layer, theproportion of the carbon black should preferably be at most 5% by weightwith respect to the weight of the ferromagnetic particles. The carbonblack has antistatic effects, friction coefficient reducing effects,light blocking property imparting effects, and film strength improvingeffects on the cleaning layer. These effects vary in accordance with thekind of the carbon black used. Therefore, in the cleaning medium inaccordance with the present invention, the kind of the carbon black, theproportion of the carbon black, and the combination of different carbonblack materials may be varied between the cleaning layer and the lowercoating layer, and the carbon black having specific characteristics,such as the particle size, the oil absorption, the electric conductance,and the pH value, may be selected in accordance with the characteristicsof the cleaning layer or the lower coating layer which is to beobtained.

The cleaning layer may also contain abrasive particles as the inorganicparticles. Examples of the materials for the abrasive particles includeα-alumina having a degree of alphatization of at least 90%, β-alumina,silicon carbide, chromium oxide, cerium oxide, α-iron oxide, corundum,artificial diamond, silicon nitride, titanium carbide, titaniumoxide,silicon dioxide, andboronnitride. Principally, one of theabove-enumerated abrasive particle materials having a Mohs hardness ofat least 6 may be used alone, or two or more of them may be used incombination. Also, a composite material obtained from theabove-enumerated abrasive particle materials (i.e, a composite materialobtained by treating the surfaces of abrasive particles with differentabrasive particles) may be used. The abrasive particles may also containcompounds or elements other than the main constituent. In such cases, ifthe proportion of the main constituent is at least 90%, the same effectscan be obtained as when the abrasive particles do not contain the othercompounds or elements. Examples of the abrasive particles includeAKP-20, AKP-30, AKP-50, HIT-50, HIT-60, HIT-60A, HIT-70A, HIT-80,HIT-80G, and HIT-100, which are supplied by Sumitomo Chemical Co., Ltd.;G5, G7, and S-1, which are supplied by Nippon Chemical Industrial Co.,Ltd.; and TF-100, and TF-140, which are supplied by Toda Kogyo K.K.

The particle size of the abrasive particles should preferably fallwithin the range of 0.01 μm to 2 μm. When necessary, abrasive particleshaving a certain particle size and abrasive particles having a differentparticle size may be used in combination. Alternatively, in cases whereabrasive particles having a certain particle size are employed, theparticle diameter distribution of the abrasive particles may be set tobe wide, and the same effects as those obtained by combining theabrasive particles having different particle sizes may thereby beobtained. The tap density of the abrasive particles should preferablyfall within the range of 0.3 g/cc to 2 g/cc. The water content of theabrasive particles should preferably fall within the range of 0.1% byweight to 5% by weight. The pH value of the abrasive particles shouldpreferably fall within the range of 2 to 11, and the specific surfacearea of the abrasive particles should preferably fall within the rangeof 1 m² /g to 30 m² /g. The abrasive particles may have an acicularshape, a spherical shape, or a dice-like shape. The abrasive particleshaving angles at portions of the shape have good cleaning properties andare therefore preferable. The proportion of the abrasive particles,which are contained as the inorganic particles in the cleaning layer,may fall within the range of 1% by weight to 45% by weight with respectto 100% by weight of the aforesaid ferromagnetic particles. In thecleaning medium in accordance with the present invention, the kind ofthe abrasive particles, the proportion of the abrasive particles, andthe combination of different abrasive particle materials may be variedbetween the cleaning layer and the lower coating layer in accordancewith the characteristics of the cleaning layer or the lower coatinglayer which is to be obtained. The abrasive particles may firstly bedispersed in the binder and may thereafter be added to the magneticcoating composition. The number of the abrasive particles, which arelocated on the surface and the side faces of the cleaning layer of thecleaning medium in accordance with the present invention shouldpreferably be at least 5 pieces/100 μm².

As the additives contained in the lower coating layer or the cleaninglayer of the cleaning medium in accordance with the present invention,the additives having the lubricating effects, the antistatic effects,the dispersing effects, the plasticizing effects, or the like, areemployed. Examples of the additives include molybdenum disulfide;tungsten disulfide; graphite; boron nitride; graphite fluoride; asilicone oil; a silicone having a polar group; a fatty acid-modifiedsilicone; a fluorine-containing silicone; a fluorine-containing alcohol;a fluorine-containing ester; a polyolefin; a polyglycol; analkylphosphoric ester and its alkali metal salt; an alkylsulfuric esterand its alkali metal salt; a polyphenyl ether; a fluorine-containingalkylsulfuric ester and its alkali metal salt; a monobasic fatty acidhaving 10 to 24 carbon atoms (which fatty acid may have an unsaturatedbond or maybe branched) and its metal salt (with Li, Na, K, Cu, or thelike); a monohydric, dihydric, trihydric, tetrahydric, pentahydric, orhexahydric alcohol having 12 to 22 carbon atoms (which alcohol may havean unsaturated bond or may be branched); an alkoxy alcohol having 12 to22 carbon atoms; a mono-fatty acid ester, a di-fatty acid ester, or atri-fatty acid ester of a mono basic fatty acid having 10 to 24 carbonatoms (which fatty acid may have an unsaturated bond or may be branched)with a monohydric, dihydric, trihydric, tetrahydric, pentahydric, orhexahydric alcohol having 2 to 12 carbon atoms (which alcohol may havean unsaturated bond or may be branched); a fatty acid ester of amono-alkyl ether of an alkylene oxide polymer; a fatty acid amide having8 to 22 carbon atoms; and an aliphatic amine having 8 to 22 carbonatoms.

Specifically, examples of the additives include lauric acid, myristicacid, palmitic acid, stearic acid, behenic acid, butyl stearate, oleicacid, linolic acid, linolenic acid, elaidic acid, octyl stearate, amylstearate, isooctyl stearate, octyl myristate, butoxyethyl stearate,anhydrosorbitan monostearate, anhydrosorbitan distearate,anhydrosorbitan tristearate, oleyl alcohol, and lauryl alcohol. It isalso possible to use nonionic surface active agents, such as an alkyleneoxide compound, a glycerin compound, a glycidol compound, and an adductof an alkyl phenol with ethylene oxide; cationic surface active agents,such as a cyclic amine, an ester amide, a quaternary ammonium salt, ahydantoin derivative, a heterocyclic compound, a phosphonium compound,and a sulfonium compound; anionic surface active agents containingacidic groups, such as a carboxylic acid group, a sulfonic acid group, aphosphoric acid group, a sulfuric ester group, and a phosphoric estergroup; and amphoteric surface active agents, such as an amino acid, anamino sulfonic acid, a sulfate or a phosphate of an amino alcohol, andan alkyl betaine compound. The above-enumerated surface active agentsare described in, for example, "Surface Active Agent Handbook", SangyoTosho K.K. The lubricating agents, the antistatic agents, and the like,need not necessarily be perfectly pure and may contain impurities, suchas isomers, unreacted materials, side reaction products, decompositionproducts, and oxides, besides the main constituents. The proportions ofthese impurities should preferably be at most 30%, and should morepreferably be at most 10%.

When necessary, the kinds and the proportions of the lubricating agents,the surface active agents, and the like, may be varied between the lowercoating layer and the cleaning layer. For example, fatty acids havingdifferent melting temperatures may be employed in the lower coatinglayer and the cleaning layer, and the bleeding to the surface of thecleaning medium may thereby be controlled. Alternatively, esters havingdifferent boiling temperatures or different levels of polarity may beemployed in the lower coating layer and the cleaning layer, and thebleeding to the surface of the cleaning medium may thereby becontrolled. As another alternative, the proportion of the surface activeagent may be adjusted such that the stability of the coating may beenhanced. As a further alternative, the amount of the lubricating agentadded to the lower coating layer may be set to be large, and thelubricating effects may thereby be enhanced.

The addition of the entire portion or a portion of each additive may becarried out at any stage of the process for producing the coatingcomposition. For example, the additive may be mixed with the inorganicparticles before the kneading process is carried out. Alternatively, theadditive may be added during the process for kneading the inorganicparticles, the binder, and the solvent together. As another alternative,the additive may be added during or after the dispersing process. As afurther alternative, the additive may be added immediately before theapplication of the coating composition. Also, in accordance with thedesired layer characteristics, after the composition for the formationof the cleaning layer is applied onto the lower coating layer, a portionor the entire portion of the additive may be applied with thesimultaneous or sequential coating technique, and the desired layercharacteristics may thereby be obtained. Further, after a calenderingprocess is carried out, or after a slitting process is carried out, thelubricating agent may be coated on the surface of the cleaning layer.

The lubricants, which may be employed in the cleaning medium inaccordance with the present invention, are available under the tradenames of, for example, NAA-102, NAA-415, NAA-312, NAA-160, NAA-180,NAA-174, NAA-175, NAA-222, NAA-34, NAA-35, NAA-171, NAA-122, NAA-142,NAA-160, NAA-173K, Castor Oil-Hardened Fatty Acid, NAA-42, NAA-44,Cation SA, Cation MA, Cation AB, Cation BB, Nymeen L-201, Nymeen L-202,Nymeen S-202, Nonion E-208, Nonion P-208, Nonion S-207, Nonion K-204,Nonion NS-202, Nonion NS-210, Nonion HS-206, Nonion L-2, Nonion S-2,Nonion S-4, Nonion O-2, Nonion LP-20R, Nonion PP-40R, Nonion SP-60R,Nonion OP-80R, Nonion OP-85R, Nonion LT-221, Nonion ST-221, NonionOT-221, Monogly MB, Nonion DS-60, Anon BF, Anon LG, Butyl Stearate,Butyl Laurate, and Erucic Acid, which are supplied by Nippon Oil & FatsCo., Ltd.; Oleic Acid, which is supplied by Kanto Kagaku K.K.; FAL-205and FAL-123, which are supplied by Takemoto Yushi K.K.; Njlub LO, NjlubIPM, and Sansocizer E4030, which are supplied by New Japan Chemical Co.,Ltd.; TA-3, KF-96, KF-96L, KF96H, KF410, KF420, KF965, KF54, KF50, KF56,KF907, KF851, X-22-819, X-22-822, KF905, KF700, KF393, KF-857, KF-860,KF-865, X-22-980, KF-101, KF-102, KF-103, X-22-3710, X-22-3715, KF-910,and KF-3935, which are supplied by Shin-Etsu Chemical Co., Ltd.; ArmidP, Armid C, and Armoslip CP, which are supplied by Lion Akzo Co., Ltd.;Duomin TDO, which is supplied by Lion Corp.; BA-41G, which is suppliedby The Nisshin Oil Mills, Ltd.; Profan 2012E, Newpol PE61, Ionet MS-400,Ionet MO-200, Ionet DL-200, Ionet DS-300, Ionet DS-1000, and IonetDO-200, which are supplied by Sanyo Chemical Industries Ltd.

Organic solvents may be used in any proportion in the lower coatinglayer and the cleaning layer of the cleaning medium in accordance withthe present invention. Examples of the organic solvents include ketones,such as acetone, methyl ethyl ketone, methyl isobutyl ketone, diisobutylketone, cyclohexanone, and isophorone; alcohols, such as methyl alcohol,ethyl alcohol, propyl alcohol, butyl alcohol, isobutyl alcohol,isopropyl alcohol, and methylcyclohexanol; esters, such as methylacetate, butyl acetate, isobutyl acetate, isopropyl acetate, ethyllactate, and glycol acetate; glycol ethers, such as tetrahydrofuran,ethylene glycol dimethyl ether, ethylene glycol monoethyl ether, anddioxane; aromatic hydrocarbons, such as benzene, toluene, xylene,cresol, and chlorobenzene; chlorinated hydrocarbons, such as methylenechloride, ethylene chloride, carbon tetrachloride, chloroform, ethylenechlorohydrin, and dichlorobenzene; N,N-dimethylformamide; and hexane.The organic solvents need not necessarily be perfectly pure and maycontain impurities, such as isomers, unreacted materials, side reactionproducts, decomposition products, oxides, and water, besides the mainconstituents. The proportions of these impurities should preferably beat most 30%, and should more preferably be at most 10%.

The same kind of organic solvent should preferably be employed in thecleaning layer and the lower coating layer of the cleaning medium inaccordance with the present invention. However, the proportions of theorganic solvent may be varied between the cleaning layer and the lowercoating layer. A solvent (such as cyclohexane or dioxane) having a highsurface tension should preferably employed in the lower coating layersuch that the stability of the coating may be enhanced. Specifically, itis important that the arithmetic mean value of the solvent compositionemployed in the cleaning layer is not smaller than the arithmetic meanvalue of the solvent composition employed in the lower coating layer. Inorder for the dispersion properties to be enhanced, the polarity of thesolvent should preferably be comparatively high. A solvent having apermittivity falling within the range of 15 to 20 should preferably becontained in the solvent composition and in a proportion of at least 50%by weight. The solubility parameter should preferably fall within therange of 8 to 11.

The thickness of the non-magnetic substrate of the cleaning medium inaccordance with the present invention may be as thin as 2.0 μm to 10 μm.The total thickness of the cleaning layer and the lower coating layermay be 1/100 to 2 times as thick as the thickness of the non-magneticsubstrate. An adhesive layer may be formed between the non-magneticsubstrate and the lower coating layer such that the adhesion betweenthem may be enhanced. The thickness of the adhesive layer may fallwithin the range of 0.01 μm to 2 μm, and should preferably fall withinthe range of 0.02 μm to 0.5 μm. A back coating layer may be formed onthe surface of the non-magnetic substrate on the side opposite to thecleaning layer. The thickness of the back coating layer may fall withinthe range of 0.1 μm to 2 μm, and should preferably fall within the rangeof 0.3 μm to 1.00 μm. As the materials for the adhesive layer and theback coating layer, any of known materials may be employed.

As the non-magnetic substrate of the cleaning medium in accordance withthe present invention, any of known films having a micro Vickershardness of at least 75 kg/mm² and having been subjected to biaxialorientation may be employed. Examples of the materials for thenon-magnetic substrate include a polyethylene terephthalate, apolyethylene naphthalate, a polyamide, a polyimide, a polyamide-imide,an aromatic polyamide, and a polybenzoxazole. In particular, thenon-magnetic substrate should preferably be constituted of an aramidresin, a polyethylene naphthalate, or a polyethylene terephthalate. Thenon-magnetic substrate may be subjected to corona discharge treatment,plasma treatment, adhesion facilitating treatment, heat treatment,dust-resistant treatment, or the like.

In order for the objects of the present invention to be accomplished,the surface roughness Ra, expressed in terms of arithmetic meandeviation, of the surface of the non-magnetic substrate, on whichsurface the cleaning layer is to be formed, should fall within the rangeof 0.5 nm to 7 nm. Thus the value of the surface roughness Ra of thesurface of the non-magnetic substrate should be small, and thenon-magnetic substrate should preferably be free from coarse protrusionsof 1 μm or larger. Also, the roughness form of the surface of thenon-magnetic substrate can be controlled freely by the size and theamount of fillers, which are added to the non-magnetic substrate whennecessary. Examples of the fillers include oxides and carbonates of Al,Ca, Si, and Ti, which may be crystalline or amorphous, and fine organicpowder, such as an acrylic type of fine powder and a melamine type offine powder. Further, such that the durability against the movementoperation may be kept high, the roughness of the surface of thenon-magnetic substrate, on which surface the back coating layer isformed, should preferably be rougher than the roughness of the surfaceof the non-magnetic substrate, on which surface the cleaning layer isformed. The surface roughness Ra of the surface of the non-magneticsubstrate, on which surface the back coating layer is formed, shouldpreferably be at least 1 nm, and should more preferably be at least 4nm. In cases where the roughness of the surface of the non-magneticsubstrate, on which surface the cleaning layer is formed, and theroughness of the surface of the non-magnetic substrate, on which surfacethe back coating layer is formed, are to be set to be different fromeach other, a substrate having a dual constitution may be employed, or acoating layer may be overlaid upon the substrate surface.

The F-5 value of the non-magnetic substrate along the direction of thetape movement (MD direction), i.e. along the longitudinal direction ofthe tape, should preferably fall within the range of 10 kg/mm² to 50kg/mm². The F-5 value of the non-magnetic substrate along the widthdirection of the tape (TD direction) should preferably fall within therange of 10 kg/mm² to 30 kg/mm². Ordinarily, the F-5 value of thenon-magnetic substrate along the longitudinal direction of the tape isset to be larger than the F-5 value of the non-magnetic substrate alongthe width direction of the tape. However, in cases where it is necessaryfor the strength of the non-magnetic substrate along the width directionof the tape to be set to be particularly large, the F-5 value of thenon-magnetic substrate along the width direction of the tape may be setto be larger than the F-5 value of the non-magnetic substrate along thelongitudinal direction of the tape. The degree of heat shrinkage, at100° C.×30 min, of the non-magnetic substrate along each of thedirection of the tape movement and the width direction of the tapeshould preferably be at most 3%, and should more preferably beat most1.5%. Also, the degree of heat shrinkage, at 80° C.×30 min, of thenon-magnetic substrate along each of the direction of the tape movementand the width direction of the tape should preferably be at most 1%, andshould more preferably be at most 0.5%. The breaking strength of thenon-magnetic substrate along each of the direction of the tape movementand the width direction of the tape should preferably fall within therange of 5 kg/mm² to 100 kg/mm². The Young's modulus of the non-magneticsubstrate should preferably fall within the range of 100 kg/mm² to 3,000kg/mm². The light transmittance with respect to light having awavelength of 900 nm should preferably be at most 30%, and should morepreferably be at most 3%.

Each of the processes for producing the coating compositions for formingthe lower coating layer and the cleaning layer of the cleaning medium inaccordance with the present invention comprises at least a kneadingprocess, a dispersing process, and mixing processes, which may becarried, when necessary, before and after the kneading process and thedispersing process. Each of the processes may be carried out in two ormore steps. The addition of each of the raw materials, such as theferromagnetic particles, the binder, the carbon black, the abrasiveparticles, the antistatic agent, the lubricating agent, and the solvent,may be carried out at the initial stage or the intermediate stage of anarbitrary process. Also, the entire amount of each of the raw materialsmay be divided into two or more portions, and the two or more portionsof the raw material may be added in two or more different processes. Forexample, the entire amount of a polyurethane may be divided into threeportions, and the three portions of the polyurethane may be addedrespectively in the kneading process, the dispersing process, and themixing process which is carried out for adjusting the viscosity of thecoating composition after the dispersing process.

In order to accomplish the objects of the present invention, aconventional production technique may be utilized at part of eachprocess. In the kneading process, a kneader having a strong kneadingpower, such as a continuous kneader or a pressure kneader, shouldpreferably be utilized. For example, in cases where the continuouskneader or the pressure kneader is utilized, the ferromagnetic particlesand the entire amount of the binder or a portion of the binder(preferably, at least 30% of the entire amount of the binder) inproportions falling within the range of 15 to 500 parts per 100 parts ofthe ferromagnetic particles are subjected to the kneading process. Thekneading techniques are described in detail in, for example, JapaneseUnexamined Patent Publication Nos. 64(1989)-79274 and 1(1989)-166338. Incases where the non-magnetic coating composition for forming the lowercoating layer is prepared, a dispersing medium having a high specificgravity should preferably be utilized. As the dispersing medium,zirconia beads are preferable.

By way of example, as the apparatus and the method for forming thecleaning medium having the dual-layer constitution in accordance withthe present invention, one of the techniques described below may beemployed.

(1) The lower coating layer is firstly applied onto the substrate byusing a gravure coater, a roll coater, a blade coater, or an extrusioncoater, which is ordinarily employed for the application of a magneticcoating composition. While the lower coating layer is being wet, thecleaning layer is applied onto the lower coating layer by using one ofsubstrate press types of extrusion coaters, which are disclosed in, forexample, Japanese Patent Publication No. 1(1989)-46186, and JapaneseUnexamined Patent Publication Nos. 60(1985)-238179 and 2(1990)-265672.

(2) The lower coating layer and the cleaning layer are appliedapproximately simultaneously by using a single coating head, which hastwo coating composition passing slits and is disclosed in, for example,Japanese Unexamined Patent Publication No. 63(1988)-88080,2(1990)-17971, or 2(1990)-265672.

(3) The lower coating layer and the cleaning layer are appliedapproximately simultaneously by using an extrusion coater provided witha back-up roll, which is disclosed in, for example, Japanese UnexaminedPatent Publication No. 2(1990)-174965.

Such that the magnetic particles may be prevented from agglomerating,shearing force should preferably be imparted to the coating composition,which is located in the coating head, with one of methods disclosed in,for example, Japanese Unexamined Patent Publication Nos. 62(1987)-95174and 1(1989)-236968. Also, as for the viscosity of the coatingcomposition, the viscosity range disclosed in, for example, JapaneseUnexamined Patent Publication No. 3(1991)-8471 should be satisfied.

In order for the cleaning medium in accordance with the presentinvention to be obtained, an orientating process may be carried out onthe ferromagnetic particles contained in the cleaning layer. Theorientating process should preferably be carried out by utilizing asolenoid having a magnetic force of at least 1,000G and a cobalt magnethaving a magnetic force of at least 2,000G, which are located such thatthe same poles may stand facing each other. Such that the state oforientation of the ferromagnetic particles after being dried may becomeas good as possible, an appropriate drying process should preferably becarried out before the orientation process is carried out.

Before the non-magnetic lower coating layer and the cleaning layer areformed with the simultaneous dual-layer coating technique, an adhesivelayer containing a polymer as the main constituent should preferably beformed, or a known technique for enhancing the adhesion with coronadischarge, UV irradiation, or EB irradiation should preferably becarried out.

When necessary, a calendering process may be carried out in order toadjust the surface roughness. As the calendering rolls, heat-resistantplastic rolls constituted of an epoxy resin, a polyimide resin, apolyamide resin, a polyimide-amide resin, or the like, may be employed.It is also possible to employ metal rolls. The calendering temperatureshould preferably fall within the range of 15° C. to 50° C. The linearpressure should preferably fall within the range of 5 kg/m to 100 kg/m,and the speed should preferably fall within the range of 50 m/min to 400m/min.

The coefficient of friction of the cleaning layer surface and theopposite surface of the cleaning medium in accordance with the presentinvention with respect to SUS420J should preferably fall within therange of 0.1 to 0.5, and should more preferably fall within the range of0.2 to 0.3. The surface resistivity should preferably fall within therange of 10⁴ to 10¹² ohms/sq. The modulus at 0.5% elongation of thecleaning layer along each of the direction of the tape movement and thewidth direction of the tape should preferably fall within the range of100 kg/mm² to 2,000 kg/mm². The breaking strength should preferably fallwithin the range of 1 kg/cm² to 30 kg/cm².

The cleaning medium as a whole should preferably be formed such that theYoung's modulus along the longitudinal direction (MD direction) may fallwithin the range of 300 kg/mm² to 1,200 kg/mm², such that the Young'smodulus along the width direction (TD direction) may fall within therange of 200 kg/mm² to 1,200 kg/mm², and such that the ratio of theYoung's modulus along the longitudinal direction to the Young's modulusalong the width direction may be between 1:2 and 2:1. The residualelongation of the cleaning medium should preferably be at most 0.5%. Thedegree of heat shrinkage at every temperature not higher than 100° C.should preferably be at most 1%, should more preferably be at most 0.5%,should most preferably be at most 0.1%, and should ideally be 0%. Theglass transition temperature (i.e., the temperature at which the lossmodulus in dynamic visco-elasticity measurement carried out at 110 Hz ismaximum) of the cleaning layer should preferably fall within the rangeof 50° C. to 120° C. The glass transition temperature of the lowercoating layer should preferably fall within the range of 0° C. to 100°C. The loss tangent should preferably be at most 0.2. If the losstangent is very large, an adhesion failure will occur. The proportion ofthe residual solvent contained in the cleaning layer should preferablybe at most 100 mg/m², and should more preferably be at most 10 mg/m².The proportion of the residual solvent contained in the cleaning layershould preferably be lower than the proportion of the residual solventcontained in the lower coating layer. The void volume in each of thelower coating layer and the cleaning layer should preferably be at most50% by volume, and should more preferably be at most 40% by volume.

As described above, the cleaning layer of the cleaning medium inaccordance with the present invention has the magnetic characteristicssuch that the coercive force Hc along the direction of the tapemovement, as measured with VSM under a magnetic field of 10 kOe, mayfall within the range of 500 Oe to 3,000 Oe. Also, the squareness ratiofalls within the range of 0.6 to 0.98. The squareness ratio shouldpreferably be at least 0.80, and should more preferably be at least0.85. The squareness ratio along the two directions, which are normal tothe direction of the tape movement, should preferably be at most 80% ofthe squareness ratio along the direction of the tape movement. The SFDof the cleaning layer should preferably be at most 0.6, should morepreferably be at most 0.5, and should ideally be 0. The remanencecoercive force Hr along the longitudinal direction should preferablyfall within the range of 1,800 Oe to 3,000 Oe. The coercive force Hc andthe remanence coercive force Hr along the vertical direction shouldpreferably fall within the range of 1,000 Oe to 5,000 Oe. The surfaceroughness Ra, expressed in terms of arithmetic mean deviation, of thecleaning layer should preferably fall within the range of 1.0 nm to 7.0nm. However, the value of the surface roughness Ra of the cleaning layershould be set appropriately in accordance with the characteristics ofthe cleaning layer which is to be obtained. The RMS surface roughnessR_(RMS), as calculated with the rating using an interatomic forcemicroscope (AFM), should preferably fall within the range of 2 nm to 15nm.

The cleaning medium in accordance with the present invention comprisesthe lower coating layer and the cleaning layer. It will be understoodeasily that the physical characteristics may be varied for the differentlayers in accordance with the characteristics of the cleaning mediumwhich is to be obtained. For example, the modulus of elasticity of thecleaning layer may be set to be high such that the durability againstmovement can be enhanced, and the modulus of elasticity of the lowercoating layer may be set to be lower than the modulus of elasticity ofthe cleaning layer such that the state of contact of the cleaning mediumwith the magnetic head can be kept good. Also, the state of contact withthe magnetic head can be improved by altering the tensilizing method forthe substrate. In cases where the substrate is tensilized in thedirection, which is normal to the longitudinal direction of the tape, agood state of contact with the magnetic head can ordinarily be obtained.

EXAMPLES

The present invention will further be illustrated by the followingnonlimitative examples. In these examples, the term "parts" means partsby weight.

Example 1

As for each of the coating composition for the lower coating layer andthe coating composition for the cleaning layer, which are describedbelow, the constituents were kneaded together by using a continuouskneader and were then subjected to a dispersing process in which a sandmill was used.

    ______________________________________                                        [Coating composition for a lower coating layer]                               Non-magnetic inorganic particles                                                                        100    parts                                        TiO.sub.2 (TTO-55A supplied by Ishihara                                       Sangyo Kaisha, Ltd.)                                                          Mean primary particle diameter: 0.05 μm                                    Specific surface area (BET method): 18 m.sup.2 /g                             pH: 7                                                                         Carbon black              20     parts                                        Mean primary particle diameter: 18 mμ                                      DBP oil absorption: 80 ml/100 g                                               pH: 8.0                                                                       Specific surface area (BET method): 250 m.sup.2 /g                            Volatile content: 1.5%                                                        Vinyl chloride-vinyl acetate-vinyl alcohol                                                              12     parts                                        copolymer                                                                     Content of --N(CH.sub.3).sub.3.sup.+ C1.sup.- polar group: 5 ×          10.sup.-4 eq/g                                                                Composition ratio: 86:13:1                                                    Degree of polymerization: 400                                                 Polyester polyurethane resin                                                                            5      parts                                        Neopentyl glycol/caprolactone polyol/MDI =                                    0.9/2.6/1                                                                     Content of --SO.sub.3 Na group: 1 × 10.sup.-4 eq/g                      Butyl stearate            1      part                                         Stearic acid              1      part                                         Methyl ethyl ketone       200    parts                                        [Coating composition for a cleaning layer]                                    Fine ferromagnetic metal particles                                                                      100    parts                                        (Fe/Zn/Ni = 92/4/4)                                                           Coercive force Hc: 1,600 Oe                                                   Specific surface area (BET method): 60 m.sup.2 /g                             Crystallite size: 195A                                                        Particle size (longer axis length): 0.20 μm                                Acicular ratio: 10                                                            Saturation magnetization σS: 130 emu/g                                  Vinyl chloride copolymer  12     parts                                        Content of --SO.sub.3 Na group: 1 × 10.sup.-4 eq/g                      Degree of polymerization: 300                                                 Polyester polyurethane resin                                                                            3      parts                                        Neopentyl glycol/caprolactone polyol/MDI =                                    0.9/2.6/1                                                                     Content of --SO.sub.3 Na group: 1 × 10.sup.-4 eq/g                      α-Alumina           2      parts                                        Particle size: 0.3 μm                                                      Carbon black              0.5    part                                         Mean particle diameter: 0.10 μm                                            Butyl stearate            1      part                                         Stearic acid              2      parts                                        Methyl ethyl ketone       200    parts                                        ______________________________________                                    

As for the coating composition for the lower coating layer, 1 part of apolyisocyanate was added to the resulting dispersion. Also, as for thecoating composition for the cleaning layer, 3 parts of a polyisocyanatewere added to the resulting dispersion. Thereafter, 40 parts of butylacetate were added to each of the coating composition for the lowercoating layer and the coating composition for the cleaning layer. Eachcoating composition was then subjected to a filtrating process with afilter having a mean pore diameter of 1 μm. In this manner, the coatingcomposition for forming the lower coating layer and the coatingcomposition for forming the cleaning layer were prepared.

Thereafter, the coating composition for forming the lower coating layerand the coating composition for forming the cleaning layer were appliedonto a polyethylene terephthalate substrate by using a simultaneousdual-layer coating technique. Specifically, thickness of thepolyethylene naphthalate substrate was 7 μm, and the surface roughnessRa, expressed in terms of arithmetic mean deviation, of the substratesurface, on which the cleaning layer was to be formed, was equal to 0.01μm. Also, the coating composition for forming the lower coating layerwas applied onto the substrate and at a rate such that the dry thicknessof the lower coating layer might be 2 μm. Immediately after the coatingcomposition for forming the lower coating layer was applied onto thesubstrate, the coating composition for forming the cleaning layer wasapplied onto the coating composition for forming the lower coating layerand at a rate such that the dry thickness of the cleaning layer might be0.3 μm. While the two layers were being wet, they were subjected to anorientating process, in which a cobalt magnet having a magnetic force of3,000G and a solenoid having a magnetic force of 1,500G were used. Thetwo layers were then dried and subjected to a calendering process, inwhich a seven-stage calendering equipment provided with metal rolls andepoxy resin rolls was used. The calendering process was carried out at atemperature of 40° C. and at a speed of 200 m/min. Thereafter, abackcoating layer having a thickness of 0.5 μm was formed on the substratesurface on the side of the substrate opposite to the cleaning layer. Thethus obtained cleaning tape web was slit into a width of 8 mm, and asample of a cleaning tape for an 8 mm video device was thereby prepared.

Example 2

A sample of a cleaning tape was prepared in the same manner as that inExample 1, except that the mean primary particle diameter of thenon-magnetic inorganic particles in the lower coating layer was set tobe as large as 0.07 μm.

Example 3

A sample of a cleaning tape was prepared in the same manner as that inExample 1, except that the thickness of the cleaning layer was set to beas thick as 0.9 μm and the thickness of the lower coating layer was setto be as thin as 1.4 μm.

Comparative Example 1

A sample of a cleaning tape was prepared in the same manner as that inExample 1, except that the thickness of the cleaning layer was set to beas thick as 1.1 μm.

Comparative Example 2

A sample of a cleaning tape was prepared in the same manner as that inExample 1, except that the mean primary particle diameter of thenon-magnetic inorganic particles in the lower coating layer was set tobe as large as 0.1 μm.

Comparative Example 3

A sample of a cleaning tape was prepared in the same manner as that inExample 1, except that 100 parts of α-alumina (mean primary particlediameter: 0.3 μm) were employed as the non-magnetic inorganic particlesin the lower coating layer in lieu of titanium oxide.

Comparative Example 4

A sample of a cleaning tape was prepared in the same manner as that inExample 1, except that 100 parts of carbon black (mean primary particlediameter: 0.05 μm) were employed as the non-magnetic inorganic particlesin the lower coating layer in lieu of titanium oxide.

Example 4

A sample of a cleaning tape was prepared in the same manner as that inExample 1, except that 100 parts of α-alumina (mean primary particlediameter: 0.06 μm) were employed as the non-magnetic inorganic particlesin the lower coating layer in lieu of titanium oxide.

Example 5

A sample of a cleaning tape was prepared in the same manner as that inExample 1, except that 100 parts of SnO₂ (mean primary particlediameter: 0.05 μm) were employed as the non-magnetic inorganic particlesin the lower coating layer in lieu of titanium oxide.

Example 6

A sample of a cleaning tape was prepared in the same manner as that inExample 1, except that 100 parts of BaSO₄ (mean primary particlediameter: 0.05 μm) were employed as the non-magnetic inorganic particlesin the lower coating layer in lieu of titanium oxide.

Example 7

A sample of a cleaning tape was prepared in the same manner as that inExample 1, except that 100 parts of ZnO (mean primary particle diameter:0.05 μm) were employed as the non-magnetic inorganic particles in thelower coating layer in lieu of titanium oxide.

Example 8

A lower coating layer and a cleaning layer were formed in the mannerdescribed below, and a sample of a cleaning tape for use in digitalaudio devices was thereby prepared.

    ______________________________________                                        [Coating composition for a lower coating layer]                               ______________________________________                                        Non-magnetic inorganic particles                                                                        100    parts                                        (acicular α-Fe.sub.2 O.sub.3)                                           Longer axis length: 0.3 μm                                                 Acicular ratio: 8                                                             Carbon black              5      parts                                        Mean primary particle diameter: 20 mμ                                      Vinyl chloride copolymer  8      parts                                        containing a --SO.sub.3 Na group and an epoxy group                           Molecular weight: 45,000                                                      Polyurethane resin        5      parts                                        containing a --SO.sub.3 Na group                                              Molecular weight: 45,000                                                      Cyclohexane               100    parts                                        Methyl ethyl ketone       100    parts                                        ______________________________________                                    

The constituents listed above were subjected to a mixing and dispersingprocess in a sand mill for four hours. Thereafter, 5 parts of apolyisocyanate (Coronate L, supplied by Nippon Polyurethane K.K.), 5parts of stearic acid, and 10 parts of butyl stearate were added to theresulting dispersion. In this manner, the coating composition for thelower coating layer was prepared.

    ______________________________________                                        [Coating composition for a cleaning layer]                                    ______________________________________                                        Fine ferromagnetic metal particles                                                                      100    parts                                        Fe alloy particles (Fe/Ni/Co)                                                 Fe:Ni:Co = 92:6:2                                                             Coercive force Hc: 1,600 Oe                                                   Saturation magnetization σS: 135 emu/g                                  Longer axis length: 0.18 μm                                                Acicular ratio: 9                                                             Vinyl chloride copolymer  10     parts                                        containing a --SO.sub.3 Na group and an epoxy group                           Polyurethane resin        5      parts                                        containing a --SO.sub.3 Na group                                              Molecular weight: 45,000                                                      α-Alumina           5      parts                                        Mean particle diameter: 0.2 μm                                             Cyclohexanone             150    parts                                        Methyl ethyl ketone       150    parts                                        ______________________________________                                    

The constituents listed above were subjected to a mixing and dispersingprocess in a sand mill for six hours. Thereafter, 5 parts of apolyisocyanate (Coronate L, supplied by Nippon Polyurethane K.K.), 5parts of stearic acid, and 10 parts of butyl stearate were added to theresulting dispersion. In this manner, the coating composition for thecleaning layer was prepared.

Thereafter, the coating composition for the lower coating layer and thecoating composition for the cleaning layer were applied onto apolyethylene terephthalate substrate by using a simultaneous dual-layercoating technique, in which doctors having different gaps were used.Specifically, thickness of the polyethylene naphthalate substrate was 7μm, and the surface roughness Ra, expressed in terms of arithmetic meandeviation, of the substrate surface, on which the cleaning layer was tobe formed, was equal to 0.01 μm. The two layers having been coated onthe substrate were subjected to an orientating process with a permanentmagnet, dried, and subjected to a calendering process. The coatingthickness of the cleaning layer was 0.3 μm, and the coating thickness ofthe lower coating layer was 2.0 μm. The thus obtained web was slitted toa width of 3.81 mm, and a sample of a cleaning tape corresponding to adigital audio tape was thereby prepared.

Comparative Example 5

A sample of a cleaning tape was prepared in the same manner as that inExample 8, except that acicular α-Fe₂ O₃ having a longer axis length of0.5 μm and an acicular ratio of 10 was employed as the non-magneticinorganic particles in the lower coating layer.

Example 9

A sample of a cleaning tape was prepared in the same manner as that inExample 8, except that acicular TiO₂ having a longer axis length of 0.1μm and an acicular ratio of 5 was employed as the non-magnetic inorganicparticles in the lower coating layer in lieu of α-iron oxide.

Comparative Example 6

A sample of a cleaning tape was prepared in the same manner as that inExample 8, except that acicular TiO₂ having a longer axis length of aslarge as 1.2 μm and an acicular ratio of as large as 15 was employed asthe non-magnetic inorganic particles in the lower coating layer in lieuof α-iron oxide.

Comparative Example 7

A sample of a cleaning tape was prepared in the same manner as that inExample 8, except that the lower coating layer was not formed and onlythe cleaning layer (having a thickness of 3.3 μm) was formed on thesubstrate.

Example 10

In this example, a sample of a cleaning disk for floppy disk drive unitswas prepared. The same coating composition for the lower coating layerand the same coating composition for the cleaning layer as those inExample 8 were used.

Firstly, the coating composition for the lower coating layer was appliedonto the front surface of a polyethylene terephthalate substrate havinga thickness of 75 μm. Thereafter, while the applied coating compositionfor the lower coating layer was being in the wet state, the coatingcomposition for the cleaning layer was applied onto it. Also, the samecoating composition applying operations were carried out on the backsurface of the substrate. The dry layer thickness of the lower coatinglayer was 3 μm, and the dry layer thickness of the cleaning layer was0.3 μm. Thereafter, a calendering process was carried out, and acleaning medium web was thereby obtained. The cleaning medium web wasthen punched into a 3.5-inch disk-like piece. The punched piece was thenset in a 3.5-inch cartridge, in which liners had been located.Predetermined mechanism parts were added to the cartridge, and a sampleof a cleaning disk was thereby prepared.

Example 11

A sample of a cleaning disk was prepared in the same manner as that inExample 10, except that the coating composition for the cleaning layerwas altered to the one described below.

    ______________________________________                                        [Coating composition for a cleaning layer]                                    ______________________________________                                        Fine ferromagnetic metal particles                                                                     100    parts                                         (Co-substituted barium ferrite)                                               Mean plate diameter: 0.06 μm                                               Platy ratio: 6                                                                Specific surface area: 35 m.sup.2 /g                                          Vinyl chloride copolymer 9      parts                                         Content of --SO.sub.3 Na group: 8 × 10.sup.-5 eq/g                      Degree of polymerization: 300                                                 Fine abrasive particles  7      parts                                         (Cr.sub.2 O.sub.3)                                                            Mean particle diameter: 0.2 μm                                             Toluene                  30     parts                                         Methyl ethyl ketone      30     parts                                         ______________________________________                                    

After the constituents listed above were kneaded together forapproximately one hour in a kneader, the constituents listed below wereadded.

    ______________________________________                                        Polyester polyurethane resin                                                                          5 parts                                               Neopentyl glycol/caprolactone                                                 polyol/MDI = 0.9/2.6/1                                                        Content of --SO.sub.3 Na group: 1 × 10.sup.-4 eq/g                      Mean molecular weight: 35,000                                                 Toluene                200 parts                                              Methyl ethyl ketone    200 parts                                              ______________________________________                                    

The resulting mixture was subjected to a dispersing process forapproximately two hours in the kneader. Thereafter, the constituentslisted below were added.

    ______________________________________                                        Carbon black          5 parts                                                 (Ketjen Black EC, supplied by                                                 Ketjen Black Intemational Company)                                            Mean particle diameter: 20 to 30 mμ                                        α-Alumina       2 parts                                                 (AKP-12, supplied by                                                          Sumitomo Chemical Co., Ltd.)                                                  Mean particle diameter                                                        (longest diameter a): 0.5 μm                                               ______________________________________                                    

The resulting mixture was subjected to a dispersing process in a sandgrinder at 2,000 revolutions and for two hours. Further, theconstituents listed below were added.

    ______________________________________                                        Polyisocyanate     6 parts                                                    (Coronate L, supplied by                                                      Nippon Polyurethane K.K.)                                                     Tridecyl stearate  6 parts                                                    ______________________________________                                    

The resulting mixture was again subjected to a dispersing process in thesand grinder, and a coating composition for the cleaning layer wasthereby obtained.

As for the samples of the cleaning media having been obtained in theExamples and the Comparative Examples described above, the thickness ofthe cleaning layer, the thickness of the lower coating layer, the kindof particles, the mean particle diameter, and the like, were determined.The results shown in Tables 1 to 3 described below were obtained. Also,cleaning tests were carried out by using the respective samples, and therate of wear of the magnetic head, the magnetic head cleaning power, andthe occurrence of scratches on the cleaned magnetic head were measured.The results shown in Tables 1 to 3 were obtained.

The measured values were obtained with the methods described below.

(Measurement of thicknesses of the cleaning layer and the lower coatinglayer)

The cleaning medium was cut to a thickness of approximately 0.1 μm alongthe longitudinal direction of the cleaning medium by using a diamondcutter. The cut piece of the cleaning medium was observed with atransmission type of electron microscope and at 30,000-powermagnification, and the photograph of the cut piece was taken. Thephotograph print size was the A4 size.

Thereafter, the interfaces among the substrate, the lower coating layer,and the cleaning layer were visually determined by paying particularattention to the difference between the shape of the non-magneticinorganic particles contained in the lower coating layer and the shapeof the ferromagnetic particles contained in the cleaning layer. Theinterfaces were bordered in black. Also, the surface of the cleaninglayer was bordered in black. The distances between the border lines weremeasured by using an image processing apparatus (IBAS2, supplied byZeiss Co.). The measurement was made with respect to a plurality ofmeasurement points sampled over the range of a sample photograph lengthof 21 cm. The simple arithmetic mean value of the measured values wastaken as the thickness of each of the cleaning layer and the lowercoating layer.

(Specific surface area measured with the BET method)

The specific surface area was measured with Quantasorb (supplied by USQuantachrome Co.). After dehydration was carried out in a nitrogenatmosphere at 250° C. for 30 minutes, the measurement was made with theBET single-point method (partial pressure: 0.30).

(Magnetic characteristics)

As for the coercive force Hc, the remanent magnetic flux density Br, thesquareness ratio, the amount of magnetization, and the like, themeasurement was carried out under an external magnetic field of 10 kOeand by using a vibrating sample type of magnetic flux meter (supplied byToei Kogyo K.K.), and the values were calculated from the magnetizationcurve. The maximum magnetic flux density Bm was measured with theaforesaid method for measuring the thickness of the cleaning layer.

(Surface roughness Ra, expressed in terms of arithmetic mean deviation)

The surface roughness Ra of the surface of the medium was measured withthe MIRAU method over an area of approximately 250 μm×250 μm by usingTOPO3D (supplied by WYKO Co. The measurement wavelength wasapproximately 650 nm, and spherical surface compensation and cylindercompensation were carried out. The used device was the non-contact typeof surface texture measuring instrument, in which the measurement wascarried out with light interference.

(Particle diameters of ferromagnetic particles and non-magneticinorganic particles)

The mean particle diameter was determined by using both of a method,wherein a photograph was taken with a transmission type of electronmicroscope, and wherein the shorter axis diameter and the longer axisdiameter of the particles were directly read out from the photograph,and a method, wherein the shorter axis diameter and the longer axisdiameter of the particles were read out by tracing the photograph, whichwas taken with the transmission type of electron microscope, with animage analyzing apparatus (IBASS1, supplied by Karl Zeiss Co.).

(Crystallite size of ferromagnetic particles)

As for the γ-iron oxide ferromagnetic particles, the crystallite sizewas determined with the diffraction X-ray method and from the spread ofthe half-width of the diffracted rays with respect to each of the (4, 4,0) face and the (2, 2, 0) face. As for the metal ferromagneticparticles, the crystallite size was determined with the diffractionX-ray method and from the spread of the half-width of the diffractedrays with respect to each of the (1, 1, 0) face and the (2, 2, 0) face.

(Rate of wear of the magnetic head)

The cleaning tape was moved in a Hi-8 Deck EV-S900 device at 5° C. andrelative humidity of 80% for 10 minutes. The height of the magnetic headof the device before being cleaned with the cleaning tape and the heightof the magnetic head after being cleaned with the cleaning tape weremeasured and taken as the rate of wear of the magnetic head.

(Magnetic head cleaning power)

The output was firstly measured with a Hi-8 Deck EV-S900 device by usingHi-8 Super DC P6-120 (supplied by Fuji Photo Film Co., Ltd.). A standardtape having been prepared for causing clogging to occur was then movedin the aforesaid Hi-8 Deck EV-S900 device, and the state of the magnetichead of the device was observed with an electronic flash equipment. Whenmuch dirt had clung to the magnetic head, the movement of the standardtape was ceased, and it was confirmed that the output was zero.Thereafter, each sample of the cleaning tape was moved in the Hi-8 DeckEV-S900 device for one minute, and the output of the Hi-8 Super DCP6-120 (supplied by Fuji Photo Film Co., Ltd.) was measured with theHi-8 Deck EV-S900 device. The difference in units of dB between theoriginally measured output and the finally measured output wascalculated and taken as the value for rating the magnetic head cleaningpower.

As for the sample of the cleaning disk described above, the cleaningpower was measured in the manner described below. Specifically, in a3.5-inch floppy disk drive unit operating on MS-DOS operating system,dirt was caused to cling to the magnetic head by using an MS-DOSformatted head dirt generating disk in accordance with the standards ofFuji Photo Film Co., Ltd., such that the magnetic head could not readsignals anymore. The cleaning disk was then inserted into the floppydisk drive unit and operated for 60 seconds. Thereafter, a judgment wasmade as to whether signals can be read from a floppy disk, on which thesignals had been recorded.

As will be clear from Tables 1, 2, and 3, with the samples of Examples 1through 11 in accordance with the present invention, the rate of wear ofthe magnetic head does not become very high, no scratch occurs on thecleaned magnetic head, and good cleaning power can be obtained. Thuswith the samples in accordance with the present invention, the resultsappropriate for the cleaning medium for the magnetic head can beobtained.

                                      TABLE 1                                     __________________________________________________________________________              Sample No.                                                                                Comp.                                                                              Comp.                                                                             Comp.                                                    Ex. 1                                                                              Ex. 2                                                                             Ex. 3                                                                            Ex. 1                                                                              Ex. 2                                                                             Ex. 3                                          __________________________________________________________________________    Cleaning                                                                           Thickness                                                                          0.3  →                                                                          0.9                                                                              1.1  0.3 →                                       layer                                                                              (μm)                                                                       Kind of                                                                            Fe/Zn/Ni                                                                           →                                                                          →                                                                         →                                                                           →                                                                          →                                            particles                                                                Lower                                                                              Thickness                                                                          2.0  →                                                                          1.4                                                                              2.0  →                                                                          →                                       coating                                                                            (μm)                                                                  layer                                                                              Kind of                                                                            TiO.sub.2                                                                          →                                                                          →                                                                         →                                                                           →                                                                          α-Alumina                                     particles                                                                     Mean 0.05 0.07                                                                              0.05                                                                             →                                                                           0.1 0.3                                                 particle                                                                      dia. (μm)                                                                  Longer                                                                             --   --  -- --   --  --                                                  axis                                                                          length                                                                        (μm)                                                                       Acicular                                                                           --   --  -- --   --  --                                                  ratio                                                                    Surface roughness                                                                       4.2  4.7 5.8                                                                              6.4  6.3 6.0                                            Ra (nm)                                                                       Rate of head wear                                                                       0.1  0.1 0.2                                                                              0.5  0.6 0.5                                            (μm/10 min.)                                                               Head cleaning power,                                                                    0.2  0   0.1                                                                              -0.3 0.2 -1.2                                           output (dB)                                                                   Number of head                                                                          0    0   0  1    0   3                                              scratches                                                                     __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________              Sample No.                                                                    Comp.                                                                         Ex. 4                                                                              Ex. 4                                                                              Ex. 5                                                                            Ex. 6                                                                              Ex. 7                                                                            Ex. 8                                          __________________________________________________________________________    Cleaning                                                                           Thickness                                                                          0.3  →                                                                           →                                                                         →                                                                           →                                                                         →                                       layer                                                                              (μm)                                                                       Kind of                                                                            Fe/Zn/Ni                                                                           →                                                                           →                                                                         →                                                                           →                                                                         Fe/Ni/Co                                            particles                                                                Lower                                                                              Thickness                                                                          2.0  →                                                                           →                                                                         →                                                                           →                                                                         →                                       coating                                                                            (μm)                                                                  layer                                                                              Kind of                                                                            Carbon                                                                             α-                                                                           SnO.sub.2                                                                        BaSO.sub.4                                                                         ZnO                                                                              αFe.sub.2 O.sub.3                             particles                                                                          black                                                                              Alumina                                                             Mean 0.05 0.06 0.05                                                                             0.05 0.05                                                                             --                                                  particle                                                                      dia. (μm)                                                                  Longer                                                                             --   --   -- --   -- 0.3                                                 axis                                                                          length                                                                        (μm)                                                                       Acicular                                                                           --   --   -- --   -- 8                                                   ratio                                                                    Surface roughness                                                                       6.7  4.8  5.0                                                                              4.6  5.1                                                                              4.9                                            Ra (nm)                                                                       Rate of head wear                                                                       0.5  0.3  0.1                                                                              0.1  0.1                                                                              0.1                                            (μm/10 min.)                                                               Head cleaning power,                                                                    0.3  -0.2 -0.1                                                                             -0.2 -0.2                                                                             -0.2                                           output (dB)                                                                   Number of head                                                                          0    0    0  1    0  1                                              scratches                                                                     __________________________________________________________________________

                                      TABLE 3                                     __________________________________________________________________________              Sample No.                                                                    Comp.     Comp.                                                                             Comp.                                                           Ex. 5                                                                              Ex. 9                                                                              Ex. 6                                                                             Ex. 7                                                                             Ex. 10                                                                            Ex. 11                                        __________________________________________________________________________    Cleaning                                                                           Thickness                                                                          0.3  →                                                                           →                                                                          3.3 0.3 →                                      layer                                                                              (μm)                                                                       Kind of                                                                            Fe/Ni/Co                                                                           →                                                                           →                                                                          →                                                                          →                                                                          Ba                                                 particles                  ferrite                                       Lower                                                                              Thickness                                                                          2.0  →                                                                           →                                                                          None                                                                              3.0 →                                      coating                                                                            (μm)                                                                  layer                                                                              Kind of                                                                            αFe.sub.2 O.sub.3                                                            Acicular                                                                           →                                                                          --  αFe.sub.2 O.sub.3                                                           →                                           particles TiO.sub.2                                                           Mean --   --   --  --  --  --                                                 particle                                                                      dia. (μm)                                                                  Longer                                                                             0.5  0.1  1.2 --  0.3 →                                           axis                                                                          length                                                                        (μm)                                                                       Acicular                                                                           10   5    15  --  8   →                                           ratio                                                                    Surface roughness                                                                       5.9  3.9  6.7 7.8 4.9 4.9                                           Ra (nm)                                                                       Rate of head wear                                                                       0.5  0.1  0.8 1.2 --  --                                            (μm/10 min.)                                                               Head cleaning power,                                                                    0.2  -0.2 0.3 0.3 ◯                                                                     ◯                                 output (dB)                                                                   Number of head                                                                          0    0    0   0   --  --                                            scratches                                                                     __________________________________________________________________________

What is claimed is:
 1. A cleaning medium for cleaning a magnetic head of magnetic recording devices, consisting essentially of:i) a non-magnetic substrate, ii) a lower coating layer, which is overlaid upon the non-magnetic substrate and primarily contains a binder and non-magnetic inorganic particles dispersed in the binder, and iii) a cleaning layer, which is overlaid upon the lower coating layer and contains a binder and inorganic particles dispersed in the binder, wherein a portion of the inorganic particles are ferromagnetic particles,wherein at least 50% by weight portion of the non-magnetic inorganic particles, which are contained in the lower coating layer, is constituted of granular inorganic particles, which have a mean particle diameter of at most 0.08 μm, or acicular inorganic particles, which have a mean longer axis length falling within the range of 0.05 μm to 0.3 μm and an acicular ratio falling within the range of 3 to 20, wherein the cleaning medium is for cleaning a magnetic head of magnetic recording devices, wherein the cleaning medium is such that a rate of head wear of a magnetic head cleaned with the cleaning medium is equal to or greater than 0.1 μm/10 min and less than 0.5 μm/10 min, and wherein the non-magnetic inorganic particles, which are contained in the lower coating layer, are constituted of at least a single kind of particle selected from the group consisting of titanium oxide particles, α-iron oxide particles, barium sulfate particles, zinc oxide particles, and alumina particles.
 2. A cleaning medium as defined in claim 1 wherein the lower coating layer and the cleaning layer are formed with a wet-on-wet coating technique, in which, after the lower coating layer has been coated on the substrate, the cleaning layer is coated on the lower coating layer while the lower coating layer is in a wet state.
 3. A cleaning medium for magnetic recording devices as defined in claim 1 wherein the thickness of the cleaning layer falls within the range of 0.05 μm to 1.0 μm.
 4. A cleaning medium for magnetic recording devices as defined in claim 1 wherein the thickness of the entire cleaning medium falls within the range of 4 μm to 15 μm.
 5. A cleaning medium for magnetic recording devices as defined in claim 1 wherein the coercive force Hc of the cleaning layer falls within the range of 500 Oe to 3,000 Oe.
 6. A cleaning medium for magnetic recording devices as defined in claim 1 wherein the lower coating layer contains carbon black having a primary particle diameter falling within the range of 10 nm to 80 nm.
 7. A cleaning medium for magnetic recording devices as defined in claim 1 wherein the acicular ratio of the ferromagnetic particles contained in the cleaning layer falls within the range of 4 to
 18. 8. A cleaning medium for magnetic recording devices as defined in claim 1 wherein the value of saturation magnetization σ of the ferromagnetic particles contained in the cleaning layer falls within the range of 100 emu/g to 180 emu/g. 